1. Field of the Invention
The present invention relates to a heterocyclic compound, a light-emitting element, a light-emitting device, an electronic device, and a lighting device.
2. Description of the Related Art
As next generation lighting devices or display devices, display devices using light-emitting elements (organic EL elements) in which organic compounds are used as light-emitting substances have been developed at an accelerated pace because of their advantages of thinness, lightweightness, quick response to input signals, low power consumption, etc.
In an organic EL element, voltage application between electrodes, between which a light-emitting layer is interposed, causes recombination of electrons and holes injected from the electrodes. The recombination brings a light-emitting substance into an excited state, and the return from the excited state to the ground state is accompanied by light emission. Since the wavelength of light emitted from a light-emitting substance is peculiar to the light-emitting substance, use of different types of organic compounds as light-emitting substances makes it possible to obtain light-emitting elements which exhibit various wavelengths, i.e., various colors.
In the case of display devices which are expected to display images, such as displays, at least three-color light, i.e., red light, green light, and blue light is necessary for reproduction of full-color images.
Further, in application to lighting devices, light having wavelength components uniformly in the visible light region is ideal for obtaining a high color rendering property, but in reality, light obtained by mixing two or more kinds of light having different wavelengths is used for lighting application in many cases. It is known that, with a mixture of three-color light, i.e., red light, green light, and blue light, white light having a high color rendering property can be obtained.
Light emitted from a light-emitting substance is peculiar to the substance, as described above. However, important performances as a light-emitting element, such as lifetime, power consumption, and even emission efficiency, are not only dependent on a light-emitting substance but also greatly dependent on layers other than a light-emitting layer, an element structure, properties of an emission center substance and a host material, compatibility between them, carrier balance, and the like. Therefore, it is true that many kinds of light-emitting element materials are necessary for the growth of this field. For the above-described reasons, light-emitting element materials with a variety of molecular structures have been proposed.
As is generally known, the generation ratio of a singlet excited state to a triplet excited state in a light-emitting element using electroluminescence is 1:3. Therefore, a light-emitting element in which a phosphorescent material capable of converting the triplet excited state to light emission is used as an emission center substance can theoretically realize higher emission efficiency than a light-emitting element in which a fluorescent material capable of converting the singlet excited state to light emission is used as an emission center substance.
However, since the triplet excited state of a substance is at a lower energy level than the singlet excited state of the substance, a substance that emits phosphorescence has a larger band gap than a substance that emits fluorescence when the emissions are at the same wavelength.
As a substance serving as a host material in a host-guest type light-emitting layer or a substance contained in each transport layer in contact with a light-emitting layer, a substance having a larger band gap or higher triplet excitation energy (energy difference between a triplet excited state and a singlet ground state) than an emission center substance is used for efficient conversion of excitation energy to light emission from the emission center substance.
Therefore, a host material and a carrier-transport material each having an extremely large band gap are necessary in order to obtain fluorescence efficiently. There are however not many variations of materials that have a sufficiently large band gap in addition to good characteristics as a light-emitting element material, and as described above, the performance of a light-emitting element depends also on the compatibility between substances. In consideration of the above, it is difficult to say that there are sufficient variations of materials with which light-emitting elements having good characteristics can be manufactured.
Furthermore, since singlet excitation energy (an energy difference between a ground state and a singlet excited state) is higher than triplet excitation energy, a substance that has high triplet excitation energy also has high singlet excitation energy. Therefore, the above substance that has high triplet excitation energy is also effective in a light-emitting element using a fluorescent compound as a light-emitting substance.
Studies have been conducted on a compound having a dibenzo[f,h]quinoxaline ring, which are examples of the host material used when a phosphorescent compound is a guest material (e.g., see Patent Documents 1 and 2).